Imagine you and your friends are at an air show. You've been waiting all day to
catch a glimpse of the famous SR-71 Blackbird, the fastest plane in the world.
The announcer reports that it's now overhead—but you can't see it because it's
flying so fast! After about a minute, you hear an enormous "BOOM," louder than
any thunder or fireworks you've ever heard! What do you think made the noise